The present invention relates to an oscillometric noninvasive method and apparatus for measuring blood pressure values and, more particularly, for determining mean, systolic, and diastolic arterial pressures.
The oscillometric method for measuring blood pressure is one of the most popular methods in commercially available systems. This method relies on measuring changes in arterial counterpressure, such as imposed by the inflatable cuff which is controllably relaxed or inflated. The cuff pressure is reduced in predetermined increments, and at each pressure level, fluctuations are monitored. The resulting signals typically consist of the DC voltage with a small superimposed variational component caused by arterial blood pressure pulsations. After suitable filtering to reject the DC voltage component and to provide amplification, peak pulse amplitudes above a given base line are measured and stored. As deflating of the cuff continues, the peak amplitudes will normally increase from a lower level to a relative maximum, and thereafter will decrease. The lowest cuff pressure at which the oscillation has a maximum value is representative of mean arterial pressure. Systolic and diastolic pressures can be derived as a predetermined fraction of mean arterial pressure, or by more sophisticated methods of direct processing of the oscillatory complexes.
One problem of such well-known oscillometric methods and apparatus is that in fact the mean pressure is not disposed exactly at the maximum peak amplitude. Another problem is that neither the systolic nor the diastolic pressures are disposed exactly at an oscillation amplitude. Therefore, the systolic, mean and diastolic values are inaccurate and differ up to a half size of a deflation step.
A further problem is in determining the mean if artifacts are recognized; said artifacts may cause an incorrect array of peak amplitudes in the area of the mean. For example, artifacts may be caused by movements of the subject during measuring. The transition from the increasing branch of an envelope of the peak amplitude sequence to the decreasing branch of said envelope is, therefore, not smooth. For example, sometimes in the transition are of increasing and decreasing branches two amplitudes of high intensity are obtained, having a less intensive amplitude therebetween. The resulting wave-shaped envelope in the mean area does not allow a determination of the mean.
Artifacts in the area of the systolic and diostolic values may be compensated by interpolation as described within EP-A2-0 207 806 and EP-A2 0 207 807. However, based on the fact that the contact between the cuff and the limb of the subject is not strong during the diastolic pressure interval, such artifacts may cause the peak amplitudes within the diastolic pressure area at the decreasing branch of the envelope to be incorrect or not obtainable.
It is, therefore, an object of the present invention to provide a method and an apparatus for oscillometric blood pressure measurings enabling the determination of mean, diastolic and/or systolic pressure values, even though artifacts occur.